We investigated the effect of increased ocular pressure in the choroid and optic nerve circulation using the heated thermocouple principle to measure heat conductivity and therefore obtain valid measurements of blood flow. With this technique we have confirmed the conclusions drawn from thermocouple measurements, namely that the circulation of the extraocular portion of the optic nerve is sensitive to increased ocular pressure and behaves like an active circulation, whereas choroid circulation, sensitive to increased pressure, behaves purely as passive circulation. These studies were performed on the cat and the Rhesus monkey. Studies aimed at elucidating this mechanism involved studying the effect of occlusion of the central retinal artery and of the short posterior ciliary arteries and measuring their contribution. The effect of autonomic drugs was also investigated.